1. Field of the Invention
The present invention refers to a process for obtaining permanently self-extinguishing cellular materials, which have a low density and high mechanical characteristics. More particularly the present invention refers to a process for obtaining cellular materials, characterized by permanent self-extinguishing properties and by low density correlated with high mechanical characteristics, by mechanical foaming of particular brominated unsaturated polyester resins. The present invention further refers to the cellular materials thus prepared, as well as to products obtained from said cellular materials.
2. Prior Art
Cellular materials essentially comprising unsaturated polyester resins having a low density are already known in the art. They are normally obtained starting from a composition of matter comprising liquid unsaturated polyester resins and various additives such as cross-linking agents, antioxidants, surfactants, foams stabilizers, nucleating agents, cell dimensions controlling agents, etc. According to the known art said compositions of matter are formed until low density liquid foams are obtained. Said foaming is carried out either by mechanical incorporation of air or other gases, or by the foaming action of expanding liquids, i.e. low boiling substances which are brought to a temperature above their boiling point, or by addition of chemical compounds which decompose when heated or as a consequence of chemical reactions, whereby to evolve gases which remains entrapped within the liquid resin.
Said liquid foams are transformed, still according to the known art, into solid cellular materials by crosslinking by means of crosslinking agents incorporated for this purpose in the starting resin.
Said cellular materials may have a density within a broad range from about 1 kg per liter to 10 gr per liter, according to the foaming method and the amount of foaming agent introduced.
The mechanical characteristics of such foams, in particular their resistance to compression, considerably vary as a function of the density, and increase as the density of the cellular material increases. Thus, it is desired to obtain possibly low density, so as to reduce the specific costs of the material and to improve the characteristics of resistance to the transmission of sound and of heat, i.e. of sound and heat insulation. In order to obtain cellular materials having high resistance to compression, in particular a resistance higher than 50 kg per cm.sup.2, but a density which is low enough to obtain the aforesaid characteristics, it is necessary to adopt a compromise between the density of the cellular material and its resistance to compression. For practical purposes, however, only cellular materials having densities below 700 gr per liter and above 300 gr per liter, to which FIG. 1 refers, are of interest. To achieve an optimal balance of the aforesaid desirable properties, it is necessary to select particular types of unsaturated polyester resin.
The solid cellular materials made of conventional unsaturated polyester resin do not have self-extinguishing characteristics, i.e. if they are subjected e.g. to a self-extinguishing test according to norm ASTM 3014-74, which requires the application of a flame under the tested material for a period of time of 10 seconds, once said flame has been removed said cellular materials will continue to burn until the portion positioned above the flame has been completely or almost completely destroyed. For such reason cellular materials of this type may be defined as non-self-extinguishing. In order to pass the aforesaid self-extinguishing test, a material should spontaneously cease to burn, either immediately, or within three seconds at the most, once the priming flame has been removed. It is also required for the practical uses of the products possessing self-extinguishing properties, that said properties be permanent, i.e. that they remain unaltered with the passing of time even if the product is employed at a temperature higher than room temperature but anyway not higher than 100.degree. C.
Cellular materials essentially comprising unsaturated polyester resins characterized by permanent self-extinguishing properties, by a density comprised between 300 and 700 gr per liter, and by a resistance to compression higher than 50 kg per cm.sup.2, are now required in various industrial sectors for safety reasons. Industrial sectors in which said products are particularly required are the electrotechnical, building and transport vehicles (aircrafts, railway wagons or coaches motorcars) sectors.
Permanently self-extinguishing cellular materials of the type of those which are the object of the present invention, are new in the art since any self-extinguishing cellular materials which have been made, have either self-extinguishing characteristics which are not permanent but decrease in time during their use, or a density which is too high for their intended uses or their compression resistance to density relationship does not correspond to the above specified required values.
Thus, for instance, one of the known method for obtaining permanently self-extinguishing materials essentially comprising unsaturated polyester resins comprises the addition of finely particulated trihydrated alumina. If a product which contains a sufficient amount (more than 40% by weight of the product) of trihydrated alumina is exposed to a fire, it exhibits self-extinguishing characteristics thanks to the fact that it liberates water from the trihydrated alumina, which water moderates the temperature increase due to the external fire. Because of the high specific weight of the trihydrated alumina (2.42 kg per liter), which is much higher than that of the conventional unsaturated polyester resins (about 1.2 kg per liter) the permanently self-extinguishing cellular materials thus obtained, have an excessively high density, and anyway not lower than 650 gr per liter, unless the amount of resin used per liter of cellular material is considerably reduced, whereby the resistance to compression is lowered to values below those acceptable for the aforesaid practical use.
Another way of obtaining self-extinguishing cellular materials essentially comprising unsaturated polyester resins, consists in adding to the resin a sufficient amount of halogenated organic compounds, such as e.g. decabromo-diphenyloxide or pentabromo-ethylbenzene. However, in order to obtain the above specified self-extinguishing characteristics, it is necessary in this case to add at least 25% by weight of said brominated compounds. However, if is desired to use said additives when foaming by mechanical gas incorporation, which is the foaming method used in the present invention, it is difficult, if not impossible, to obtain densities lower than the required upper density limit that is lower than 700 gr per liter, because of the considerable amounts of said additives necessary to obtain self-extinguishing properties, because of the high specific density of the brominated organic additives, and because of the mechanical difficulties created by the presence of high quantities of solid or semi-solid additives during the foaming process. Even in the extreme case in which self-extinguishing cellular materials having a sufficiently low density could be obtained with said additives, their self-extinguishing properties would disappear with the passing of time, because of the migration of the brominated additive to the surface and its subsequent loss due to mechanical abrasion. In this way the cellular material exhibits, after a sufficiently long time, self-extinguishing properties which are too low and therefore practically no longer adequate to the practical use, and this all the more if the cellular product is employed at a temperature that is substantially higher than room temperature.
Some partially self-extinguishing cellular materials essentially comprising polyester resins, obtained by chemical foaming or by means of volatile expanding agents, are known in the art. In this case "chemical foaming" means the formation of gas within the resin due to chemical reaction, such as e.g. the decomposition of a chemical agent or due to the reaction of a chemical agent with certain functional groups of the resin. Both the chemical foaming and the foaming effected by means of volatile expanding agents (e.g. fluorinated hydrocarbons) lead, however, to considerable drawbacks, which are e.g. high cost of the chemical or expanding agents, toxicity thereof, air pollution (in the case of fluorinated hydrocarbons), instability of the chemical agent which needs to be stored at low temperature.